1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, and more particularly to a ferroelectric memory device using a ferroelectric material.
2. Description of the Prior Art
It is known in the art that a ferroelectric capacitor comprising a ferroelectric thin film sandwiched by a pair of capacitive electrodes exhibits hysteresis between the voltage applied thereto and the polarization that represents the displacement of a positive or negative electric charge caused by the applied voltage. After a certain voltage is applied to the ferroelectric capacitor to make it polarized, it remains polarized due to the hysteresis even when the applied voltage is eliminated. This property of the ferroelectric capacitor may be used to turn it into a nonvolatile memory device. A nonvolatile memory device that comprises a ferroelectric capacitor as a memory element is referred to as a ferroelectric memory device.
Ferroelectric memory devices have different memory cell types. One memory cell type is a two-transistor, two-capacitor type comprising two cell transistors and two ferroelectric capacitors. Another memory cell type is a one-transistor, one-capacitor type comprising a single cell transistor and a single ferroelectric capacitor. The two-transistor, two-capacitor type memory cells are advantageous in that they are less susceptible to variations in the fabrication process. The one-transistor, one-capacitor type memory cells are advantageous in that they lend themselves to being constructed as a highly integrated circuit.
The memory cells of these different types operate on the same basic principle that a voltage is applied to the ferroelectric capacitor to read and write data. For writing data, it is necessary to apply a voltage higher than the voltage at which the residual polarization of the ferroelectric capacitor starts being reversed, i.e., the coercive voltage at a point where the hysteresis curve crosses a linear curve of no polarization, for causing the reversed polarization to remain.
The ferroelectric capacitor is known as causing deterioration referred to as fatigue and imprinting if the polarization is repeatedly reversed. The ferroelectric capacitor suffers more fatigue and imprinting as the applied voltage is higher. If the deterioration progresses, then signal voltages applied to sense amplifiers in the ferroelectric memory device are lowered to the extent that data cannot be read and written properly. Specifically, as the applied voltage is higher, the number of times that the ferroelectric capacitor can repeatedly be operated is reduced, and the number of times that the ferroelectric memory device can write data is also reduced.
For the above reasons, it is important to apply such a voltage as to sufficiently reverse the polarization of ferroelectric capacitors and minimize the deterioration thereof in the ferroelectric memory device.
One scheme to produce such a voltage is an arrangement for controlling the applied voltage. Specifically, an external power supply voltage is lowered by a step-down circuit (voltage reducing circuit) to generate an applied voltage that is used to drive plate lines. Since word lines need to drive transistors, a voltage higher than the applied voltage used to drive the plate lines is used to drive the word lines. With such a voltage controlling arrangement, it is possible to produce a ferroelectric memory device which can sufficiently read and writ data, suffers reduced deterioration of ferroelectric capacitors, and can repeatedly be operated for an increased number of times.
Since ferroelectric memory devices are of such a structure that it can operate at a lower voltage than EEPROMs (electrically erasable and programmable read-only memories) and flash memories that are generally used as nonvolatile memories, they are expected to be used as nonvolatile memory devices for IC (integrated circuit) cards. IC cards are required to be operable in a very wide power supply voltage range. For example, contact IC cards are required to be operable at both power supply voltages of 5 V and 3 V, and contactless IC cards are required to be operable at a lower voltage.
However, the conventional ferroelectric memory devices are problematic in that if the power supply voltage is lowered, then no sufficient voltage can be applied to the ferroelectric capacitors, and as a result, no sufficient residual polarization occurs in the ferroelectric capacitors. These drawbacks are caused by the arrangement of the conventional ferroelectric memory devices which is not designed to bring about a substantial reduction in the power supply voltage.
It is therefore an object of the present invention to provide a ferroelectric memory device which is operable in a wide range of allowable power supply voltages such that it can operate at a low power supply voltage.
According to the present invention, the above object can be achieved by a ferroelectric memory device comprising a bit line, a ferroelectric capacitor for storing data based on residual polarization thereof, the ferroelectric capacitor comprising a ferroelectric thin film and a first capacitive electrode and a second capacitive electrode which sandwich the ferroelectric thin film therebetween, a cell transistor connected between the first capacitive electrode and the bit line, a first drive circuit for selectively supplying a first voltage and a ground potential to the second capacitive electrode, and a step-down circuit for reducing an external power supply voltage supplied in a predetermined voltage range into the first voltage, the ferroelectric thin film being formed such that a coercive voltage thereof is smaller than the first voltage.
Preferably, a reduced voltage generated by reducing the external power supply voltage to a minimum value of its variable range should be applied to the ferroelectric capacitor. Furthermore, the ferroelectric thin film should be formed such that a coercive voltage of the ferroelectric capacitor is equal to a minimum value of a range for allowing the ferroelectric thin film to remain sufficiently polarized.
With the above arrangement, the ferroelectric memory device is operable in a wide range of external power supply voltages, has the ferroelectric thin film that can remain sufficiently polarized to store data, and suffers reduced deterioration due to repeated operation.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.